1. Field of the Invention
The present invention generally relates to oil and gas well (borehole) logging tools, and more particularly to an improved method of measuring the density of geologic formations using pulsed neutrons to produce gamma radiation which travels through the formation, wherein the gamma diffusion length provides a measure of density.
2. Description of the Related Art
Logging tools for measuring earth formation properties are well known, particularly those used in the location of underground petroleum products (oil and gas). Many borehole logging instruments use various techniques to determine geophysical properties such as bulk density, porosity, water saturation, and gas saturation. Among these techniques are those involving the use of radiant energy. For example, gamma rays are commonly used to measure bulk density of a formation by detecting such radiation as it passes through the formation and relating the amount of detected radiation to the electron density of the formation. See, e.g., U.S. Pat. No. 4,297,575.
Gamma rays can be emitted continuously from a source in the borehole tool and propagate outward into the formation. A typical gamma ray source is cesium-137. The electron density of the formation is calculated based on the count rate or intensity of the backscattered gamma rays that are received at detectors located in the tool. Usually at least two detectors (far and near) are used, which allows a measure of formation density that is essentially independent of the mudcake surrounding the tool (the mudcake is the layer of solid material lining the open borehole that has consolidated from the drilling fluid). The detectors are shielded from direct radiation from the gamma ray source by high density material located in the tool body. The use of a gamma ray source and gamma ray detectors is referred to as gamma-gamma logging. See generally, "The Physical Foundations of Formation Density Logging (Gamma-Gamma)," Geophysics vol. XXX, no. 2 (April 1965).
These two-detector density logging tools are satisfactory for use with open, smooth boreholes, but they have a limited depth of investigation (-4" or 10.2 cm) and are not quantitative in cased holes. Cased-hole density logs are often needed to evaluate producing gas wells, and for environmental monitoring. While these instruments can compensate for mudcake, they are unable to accurately compensate for the borehole casing and cement. In particular, the steel casing is up to one-half inch thick and very dense, and thus very effective at blocking the passage of low-energy gamma rays. Various attempts have been made to overcome these limitations. One prior art technique, disclosed in U.S. Pat. No. 5,525,797, requires the use of three or more detectors, and results in improved measurements, but the need for more detectors clearly complicates the tool (and makes it more expensive). Another approach is to use a source with a higher energy level of gamma rays, such as cobalt-60 (which provides 1.173 and 1.333 MeV gamma rays instead of the 0.662 MeV gamma rays from cesium). Although this approach has some advantages, the tool will still have a relatively shallow depth of investigation.
A different approach involves detection of gamma radiation produced in the formation in response to a high-energy neutron source, referred to as induced gamma ray logging. The radiation is analyzed using one of two common techniques to determine the porosity (not density) of the formation. The two methodologies are referred to as GST (gamma spectroscopy tool) and C/O (carbon:oxygen). In the GST-type method, silica, calcium and hydrogen levels are broken out using spectral techniques, and the amount of hydrogen is compared to the combined amount of silica and calcium to determine porosity. In C/O systems, carbon and oxygen levels are determined using spectral techniques and the ratio is then related to porosity.
When the neutron source is pulsed, gamma rays are produced by one of three reactions, inelastic scattering of fast neutrons (neutrons with an energy of about one MeV or within about one order of magnitude), epithermal neutron capture (neutrons with an energy of about one eV), and thermal neutron capture (neutrons with an energy of about 0.025 eV). The fast-neutron lifetimes are very small (a few microseconds) such that during the source pulse a mixed-energy neutron field exists. Shortly after the burst, all neutrons are thermalized (slow down) and these thermal neutrons wander about until being captured, with a lifetime in the hundreds of microseconds. Gamma rays from inelastic scattering are produced in close proximity to the accelerator, and gamma rays from thermal capture are dispersed farther from the accelerator (up to tens of centimeters).
Techniques for determining porosity based on capture gammas fall into one of five basic detector types: single thermal neutron detectors; dual thermal neutron detectors; single epithermal neutron detectors; dual epithermal neutron detectors; and pulsed-neutron capture gamma-ray detectors. A single thermal neutron detector examines the slowing down lengths (amounts of hydrogen and steel in the environment) and relates the number of thermal neutrons to the amount of hydrogen, i.e., a low count rate implies more hydrogen and higher porosity. A dual thermal neutron detector is similar to a single thermal neutron detector but the former has two detectors to provide for compensation techniques for neutron absorbers and near-wellbore variations. A single epithermal neutron detector also similar to a single thermal neutron detector, but the former examines slowing down lengths to epithermal energies. A dual epithermal neutron detector again uses two detectors to provide compensation for near-wellbore variations. A pulsed-neutron capture gamma-ray detector measures the gamma rays produced by thermal neutron absorption using two detectors (far and near), and relates differences in count rates to the amount of hydrogen present.
An early GST system is shown in U.S. Pat. No. 4,055,763 which uses a pulsed fast neutron source and one detector. Thermal neutron decay times are derived which characterize the formations based on ratios of elemental constituents such as hydrogen, calcium and silicon. An early C/O logging system is disclosed in U.S. Pat. No. 3,946,226, which uses a pulsed fast neutron source and one gamma ray spectroscopy detector. Both the inelastic and capture gammas are analyzed to determine the carbon:oxygen ratio. That patent also notes that the inelastic gamma ray counting rate is to some degree dependent on the hydrogen content of the formation. Later instruments added further refinements. For example, U.S. Pat. Nos. 4,122,339 and 4,122,340 teach a method of combining the fast and epithermal neutron population measurements to provide a measurement of porosity; an organic scintillator can replace the gamma ray detector to make fast neutron measurements. The same basis for the porosity derivation is described in U.S. Pat. No. 4,605,854, but speaks in terms of the pulse height range of the fast neutron energy spectrum only. In U.S. Pat. No. 4,239,965, the logging tool examines the ratio of gamma ray energies in the capture signal to determine both carbon:oxygen and hydrogen:iron ratios.
Another device using a high energy neutron source is disclosed in U.S. Pat. Nos. 4,645,926 and 4,656,354 (and is sold under the brand name PDK-100 by Dresser Atlas, a division of Dresser Industries, Inc.). That device measures the ratio of inelastic to capture gamma rays (RIC) which is related to porosity. These patents note that heavy elements (higher density) create more inelastic gamma rays such that the higher the density, the higher the inelastic component in the ratio. The "inelastic" count rate used in actually a mix of capture and inelastic rates since no attempt is made to separate these components, so it is unclear what the basis of this porosity determination would be when the capture and inelastic gamma ray mix varies due to non-porosity factors. A method for separating the inelastic scattering events from the mixed count samples during the source pulse is taught in U.S. Pat. No. 5,374,823. A variation of the RIC is described in U.S. Pat. No. 4,430,567, specifically involving the ratio of capture events collected after the source pulse to the mixed capture and inelastic events collected during the source pulse. The '926 and '354 patents also refer to the ratio of inelastic gamma rays measured by the near and far detectors (RIN), which is used to identify wellbore geometry variations. The RIN is also used to differentiate hydrocarbon type in shaly sands, but no attempt has been made to relate RIN to porosity.
Yet another approach involving the use of high-energy neutrons relates to inelastic porosity (IPHI) which is a logarithmic function of the near detector inelastic rate, with increasing counts equal to a higher porosity. The same measurement made at the far detector is referred to as FPHI. Recorded count rates are processed to remove the capture and background components leaving only inelastic gamma rays. these inelastic count rates are then scaled in porosity as a logarithmic function of the count rate with an offset. This response is very similar to that of an open-hole density. IPHI is mentioned in "Cased Hole Exploration: Modern Pulsed Neutron Techniques for Locating By-Passed Hydrocarbons in Old Wells," Society of Petroleum Engineers (SPE) Permian Basin Oil & Gas Recovery Conference Proceedings, pp. 167-176 (March 1996); "A New 1.625" Diameter Pulsed Neutron Capture and Inelastic/Capture Spectral Combination System Provides Answers in Complex Reservoirs," SPWLA 35th Annual Logging Symposium (June 1994); and "Using Pulsed Neutron Decay-Spectrum Data and Multi-inflatable Packer Plugdown Assemblies Improve Oil Production Rate in a Mature CO.sub.2 Flood," SPE Permian Basin Oil & Gas Recovery Conference Proceedings, pp. 203-209 (March 1996). Near-wellbore variations and low hydrogen content formations can cause large variances in the gamma ray source size and strength, causing large uncertainties in the porosity calculations. This porosity is based on the fact that the gamma rays created by the inelastic scatter of fast neutrons are Compton-scattered as they make their way to the detectors where they are counted. As long as the transport length is sufficient for the attenuation of the gamma rays (by Compton scattering) to be the dominant component in the received signal, a density-type porosity can be computed. However, if the porosity, borehole size and content are such that Compton scattering is no longer the dominant component (i.e., the gamma ray transport length is short due to the gamma ray creation taking place "near" one or both detectors), then the sensitivity of the count rate to porosity can be diminished (or even inverted).
To summarize the foregoing, gamma-gamma logging provides a measure of formation bulk density (which is problematic with cased holes), while induced gamma ray logging provides a measure of formation porosity (which can also be problematic as noted). A further problem with hydrogen-based porosity measurement is that it rests on two important assumptions: that there is little or no hydrogen in a solid rock matrix, and that all hydrogen present is in the form of water located in the pores. These assumptions can fail, such as in a shale which has a high hydrogen level, or in a formation having a high porosity but filled with gas, not water. Therefore, extrapolating density from porosity is not necessarily accurate.
A hybrid technique may be used wherein induced gamma rays from a high-energy neutron source are analyzed to determine bulk density directly, instead of porosity. See "Bulk Density Logging With High-Energy Gammas Produced by Fast Neutron Reactions with Formation Oxygen Atoms," 1995 IEEE Nuclear Science Symposium (copyright 1996). Attenuation of the high-energy neutrons as they transport into the formation, and attenuation of the gammas by the material between the detectors and the gamma sources, are the bases for the density measurement. In this system, a near detector is positioned close (22 cm) to the pulsed source for monitoring the output field and a far detector is placed at a large spacing (100 cm) for density sensitivity. The measured data are the count rates during the burst period. This logging method appears to penetrate more deeply into the formation, even in the presence of borehole casing and grout. Subsequent experiments, however, have shown anomalous behavior as the formation and borehole were filled with water, an effect of the capture counts not being removed. It would, therefore, be desirable to devise a method of using induced gamma ray logging to determine bulk density instead of porosity, but which is not subject to this deficiency. It would be further advantageous if the method were less sensitive to borehole variations.